Fire safety is important because of the destructive nature of an uncontrolled fire. Fire is a self-propagating cycle of reactions where matter changes form as indicated by the visible and tangible side effects of heat, light, and gas emission. Fire is caused by raising a fuel (such as plastic or wood, for example) to its combustion temperature. The heat decomposes the complex molecules of the fuel into smaller molecules that recombine with oxygen in the air to produce different fuels, generate more heat, and release various gases. The burning fuel releases free radicals that emit light or produce a flame. The fuel decomposition and radical emission contribute to the fire cycle until only the nearly pure carbonaceous base of the fuel, known as char, remains. The fire is accompanied by the release of volatile gas compounds, or smoke, from the fuel components that do not char. As long as there is a source of oxygen, free radicals, and fuel, the fire will continue and may become uncontrollable.
Technologies such as flame retardants and flame resistant materials, for example, have been developed to control fire by raising the combustion temperature, reducing the rate of burning, reducing flame propagation, and reducing smoke generation. Flame retardants include inorganic minerals, phosphorous compounds, halogenated organic compounds, and others for the inclusion in various fuels or substrates. The inorganic minerals reduce heat generated to below the combustion temperature of the fuel by endothermically decomposing and releasing water vapor upon decomposition. The phosphorous compounds stop fire by expediting charring of the outer layers of the fuel to insulate any remaining fuel from burning and to prevent the release of combustible gases. The halogenated organic compounds stop fire by eliminating the free radicals that contribute to the combustion process.
Choosing a flame retardant may be based on the desired mechanism of protection or may be based on non-fire related criteria as demonstrated in the transportation industry where the multiple safety variables are considered in order to best protect equipment and/or passengers. In mobile platforms such as aircrafts, trains, buses, or ships that are operator driven, it is necessary that visibility through transparencies of the mobile platform such as windows does not interfere with the visual range needed to safely maneuver the mobile platform. When selecting flame retardants for the transparency, particular care must be used because upon combustion of transparency polymers, the initial polymer material burns to undergo crosslinking, aromatization, fusion of aromatics, polymer chain scission, polymer chain stripping, and the like. These changes in polymer structure prolong the burning, provide innumerable fuel sources, increase flame propagation, and increase the generation of toxic gases.
Current methods of preventing fire and enhancing fire resistance of transparencies have included incorporating phosphorous and halogenated organic compounds into the transparency material, for example, by applying coatings of the materials onto the transparencies. Despite the benefits, the phosphorous and halogenated organic compounds produce corrosive smoke and/or various environmentally detrimental and toxic emissions. Furthermore, the coated transparencies have a different refraction index (RI) than the uncoated transparency material causing light bending and image distorting through the coated transparency or cause clouding and opacity of the transparent material.
Accordingly, there is still a need for a fire-resistant transparency that can provide fire resistance, reduce flame spreading, reduce the rate of heat release, facilitate extinction of the fire, minimize smoke evolution, and does not distort the refractive index of the transparency or impair the visibility through the transparency. The present invention is illustrated in connection with an aircraft window, however it is applicable to any transparency where fire resistance and undistorted visibility are of paramount importance.